docs: def charset(self):
          """The charset from the content type."""
          header = self.environ.get('CONTENT_TYPE')
          if header:
              ct, options = parse_options_header(header)
              charset = options.get('charset')
              if charset:
                  if is_known_charset(charset):
                      return charset
                  return self.unknown_charset(charset)
          return self.default_charset